volume rendering in fragment shader calculate depth value - opengl

Question

I have a volume rendering program write by OpenGL and GLSL.Now I need calculate a fragment's depth value .The follow code is my fragment shader program
#version 120
//
some needed variable declaration
//
float calculateDepthValue(float t, float entryPointsDepth, float exitPointsDepth)
{
// assign front value given in windows coordinates
float zw_front = entryPointsDepth;
// and convert it into eye coordinates
float ze_front = 1.0 / ((zw_front - const_to_z_e_1)*const_to_z_e_2);
// assign back value given in windows coordinates
float zw_back = exitPointsDepth;
// and convert it into eye coordinates
float ze_back = 1.0 / ((zw_back - const_to_z_e_1)*const_to_z_e_2);
// interpolate in eye coordinates
float ze_current = ze_front + t*(ze_back - ze_front);
// convert back to window coordinates
float zw_current = (1.0 / ze_current)*const_to_z_w_1 + const_to_z_w_2;
return zw_current;
}
float getDepthValue(float t, float tEnd, float entryPointsDepth, float exitPointsDepth)
{
if (t >= 0.0)
return calculateDepthValue(t / tEnd, entryPointsDepth, exitPointsDepth);
else
return 1.0;
}
vec4 compositeDVR(in vec4 curResult, in vec4 color,in float t, in float pvar,inout float tDepth)
{
vec4 result = curResult;
// apply opacity correction to accomodate for variable sampling intervals
color.a = 1.0 - pow(1.0 - color.a, pvar);
result.rgb = result.rgb + (1.0 - result.a) * color.a * color.rgb;
result.a = result.a + (1.0 - result.a) * color.a;
// save first hit ray parameter for depth value calculation
if (tDepth < 0.0)
tDepth = t;
return result;
}
void main()
{
vec2 p = gl_FragCoord.xy * screenDimRCP_;
vec3 start = texture2D(frontTex, p).rgb;
vec3 end = texture2D(texBack, p).rgb;
float entryDepth = texture2D(entryPointsDepth_, p).z;
float exitDepth = texture2D(exitPointsDepth_, p).z;
//
some needed code
//
lookup = texture1D(globalTex, sample);
if (lookup.a > 0.0)
{
lookup.rgb = phongShading(N, texToPhysicalPos, position_, cPos, lookup.rgb, lookup.rgb, vec3(1.0, 1.0, 1.0));
//compositeDVR
result = compositeDVR(result, lookup, t, powAlp, tDepth);
}
gl_FragDepth = getDepthValue(tDepth, tEnd, entryDepth, exitDepth);
gl_FragColor = result;
}
In fragment shader,some function come from Voreen's code.my program's result is that gl_FragDepth all equal 1,this is wrong.any people can help me?